The 10-mile twin-tunnels, to be built by Align JV, represent a key stretch of HS2 phase one between London and Birmingham. They are the longest and deepest tunnel bores on route, with separate northbound and southbound tunnels and five ventilation shafts.
Two 2,000 tonne machines – currently being manufactured by Herrenknnecht in Germany – will dig the tunnels from a site next to the M25 near South Heath in Buckinghamshire.
At 136 acres, the South Portal Chalfont Lane site will be the biggest construction site on the project. From here, Align JV – made up of Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick – will deliver the tunnelling and the construction of the adjacent Colne Valley viaduct.
Dedicated motorway slip-roads have been built to link the new site with the M25 and take construction traffic off local roads, while material excavated from the tunnels will be reused as landscaping on site.
Around 350 staff from Align JV and its subcontractors are already permanently based on site. At peak, around 1,200 people are expected to be employed in the design and construction of the Chiltern tunnels and the viaduct.
The tunnels will carry the state-of-the-art HS2 trains under the Chiltern hills on their way to Birmingham, Scotland and the north of England.
Just south of the tunnel portal, the Colne Valley Viaduct – designed by Align, working with design partners Jacobs and Ingerop-Rendel, and architect Grimshaw – will be the longest railway viaduct in the UK. At 3.4km long, it will carry HS2 across the Grand Union Canal and surrounding lakes. Further south, the line will go into tunnel again before arriving at Old Oak Common and London Euston.
“The south portal site will have one of the most important parts to play in the story of HS2,” said HS2 delivery director David Bennett. “From here we’ll manage the construction of the Chilterns tunnel and the iconic Colne Valley viaduct.”
He added: “It’s great to see how much progress has been made on site – and I look forward to working with the team to deliver what will be two of the most exciting and ambitious engineering challenges on the project.”
Align JV project director Daniel Altier said: “Significant planning and preparation has been completed by the Align team, working in close cooperation with HS2 and it’s great to see developments at the south portal. We now have a significant presence on-site and this is set to increase over the weeks and months ahead.
"This is a complex project and it’s both rewarding and exciting for the team to see the component parts coming together after all their detailed planning. Align is now actively recruiting to build our team to address the complex engineering challenges associated with the delivery of this iconic project.”